Piezoelectric actuators are used in various applications such as, for example, in disk-storage devices for regulating a mechanical quantity in response to an electrical control quantity. A piezoelectric actuator is formed by a chip enclosed between two metal plates. If the metal plates are charged electrically, the chip is deformed resiliently as if it were subjected to a mechanical stress. The deformation of the chip depends on the previous operation of the piezoelectric actuator as well as on the charge state of the metal plates.
There is more than one relationship between the electrical control quantity and the deformation so that a characteristic curve corresponding to this relationship has a hysteresis loop. The hysteresis loop of the characteristic curve of the piezoelectric actuator is greatly dependent on the type of control used, as well as on the frequency of the control quantity.
The electrical behavior of the piezoelectric actuator may be represented as a first approximation by an equivalent circuit including a capacitor with variable capacitance. A dissipation resistor is connected in parallel to the capacitor during the generation of the mechanical stress. The piezoelectric actuator can therefore be driven by a voltage or by a charge, also referred to as the voltage mode or the charge mode.
A circuit for driving the piezoelectric actuator in the voltage mode enables a voltage applied to the chip to be controlled by a control voltage. The driver circuit, which is formed by an operational amplifier with voltage feedback, is relatively straightforward. However, this approach does not enable the deformation of the chip to be precisely controlled because of the large hysteresis loop of the voltage/deformation characteristic curve of the piezoelectric actuator.
Greater accuracy can, however, be achieved by driving the piezoelectric actuator in the charge mode since the hysteresis loop of the charge-deformation characteristic curve is generally much smaller than that of the voltage/deformation characteristic curve. A circuit for driving the piezoelectric actuator in the charge mode enables a charge transferred to the chip to be controlled by the control voltage. The driver circuit requires an amplifier which measures a current passing through the chip by detecting a voltage at the terminals of a resistor connected in series with the chip. The amplifier produces as an output a quantity indicative of the charge transferred to the chip over time by an integration operation. This quantity is transferred to the input of the drive circuit to interrupt its operation when the charge transferred to the chip reaches a desired value.
A disadvantage of the approach described above is that it requires a complex measuring amplifier, i.e., a sense amplifier. Moreover, in some applications such as, for example, when the piezoelectric actuator is disposed on a suspension arm of a disk-storage device, one plate of the piezoelectric actuator formed by the suspension arm is connected to a reference terminal, such as ground. The resistor used for measuring the current through the chip cannot be connected to ground. This makes it necessary to use a differential sense amplifier. The differential sense amplifier is difficult to produce because of the high values of the input voltage, which are on the order of several tens of volts.